1. Field of the Invention
This invention relates to a reduction motor shaft provided with screw type double worms constituting a reduction mechanism part.
2. Description of the Prior Art
As a motor having a reduction motor shaft provided with screw type double worms for the reduction mechanism, for instance, there is a wiper motor as shown in FIGS. 1 and 2. This wiper motor 11 comprises a motor yoke 12, a field magnet 13 fixed to the inner wall of the yoke 12, an armature 14 rotatably arranged inside the field magnet 13, a commutator 15 supplying an electric current to the armature 14, a reduction motor shaft 16 fixing the armature 14 and commutator 15, and a carbon brush 17 contacting with the commutator 15, in which the carbon brush 17 is attached to a fit portion between motor yoke 12 and gear housing 18. Moreover, numeral 19 is a cover for the gear housing 18.
Onto one end side of the reduction motor shaft 16 (left-hand side of FIG. 1) are coaxially formed a right-handed screw type worm 21 and a left-handed screw type worm 22 as one body. These right-handed and left-handed screw type worms 21 and 22 are double-thread screws, and tooth profiles of the worms 21 and 22 are linked with each other. Further, the worms 21 and 22 are engaged with worm wheels 23 and 24 pivoted in the gear housing 18, respectively, while pinions 23a and 24a integrally united with the worm wheels 23 and 24 are engaged with a common toothed wheel 25 pivoted in the gear housing 18 to thereby construct a reduction mechanism part 26, whereby a wiper blade (not shown) is subjected to a reciprocating oscillation or a rotating movement through a reduction shaft (not shown) connected to the wheel 25.
The reduction motor shaft 16 is fitted into an oil-containing bearing 31 arranged between the commutator 15 and the worm 22. The bearing 31 is supported by the housing 18 and a bearing support member 32.
The other end side of the reduction motor shaft 16 (right-hand side of FIG. 1) passes through an oil-containing bearing 34 retained by a bearing support member 33 and protrudes from the yoke 12. To this end of the shaft 16 is fitted a self-cooling fan 35 through a nut 36. A fan cover 37 provided with a plurality of air inlet holes 37a is fixed to the yoke 12 by means of screw bolts so as to surround the self-cooling fan 35 in such a manner a ventilation passage 39 is formed between the outer surface of the yoke 12 and the fan cover 37.
In the reduction motor shaft 16 of the above construction, the right-handed screw type worm 21 and the left-handed screw type worm 22 are coaxially formed as a double-thread screw on the reduction motor shaft 16 and also the tooth profiles of the worms 21 and 22 are linked with each other, so that a direction of thrust produced in the shaft 16 by engaging the right-handed screw type worm 21 with the worm wheel 23 is opposite to a direction of thrust produced in the shaft 16 by engaging the left-handed screw type worm 22 with the worm wheel 24, and consequently these thrusts produced in the reduction motor shaft 16 are offset with each other. As a result, it is not substantially necessary to give a function supporting the thrust to the oil-containing bearings 31 and 34, so that the shaft 16 is sufficiently simple in the structure.
In the reduction motor shaft 16 provided with conventional screw type double worms, however, when the screw of the worms 21 and 22 are formed from a threading boundary 40 by rolling, the material for the reduction motor shaft 16 is abruptly rolled in a direction from addendums 41, 42 to dedendums 51, 52 as shown in FIG. 2, so that the rolling pressure becomes excessive, which is particularly apt to cause deformation in the vicinity of the threading boundary 40. As a result, failures such as deformation, bending, breaking and the like are caused on the longitudinal axis of the reduction motor shaft 16.